Cholecalciferol has no effect on calcium and inorganic phosphorus balance in a naturally cholecalciferol-deplete subterranean mammal, the naked mole rat (Heterocephalus glaber)

Abstract

Naked mole rats, Heterocephalus glaber, have no obvious source of cholecalciferol (D3) available to them, given their underground habitat and tubiferous diet. They have undetectable levels of 25-OH-D3 and as such appear to be naturally deplete in D3. The effect of an oral D3 supplement on mineral balance and homeostasis was therefore investigated. This D3 treatment did not affect circulating levels of Ca2+ and inorganic phosphorus (P(i]. Nor did D3 treatment affect mineral intake and absorption. The Ca2+ and P(i) present in the food was efficiently extracted and absorbed, resulting in an apparent fractional absorption (AFA) efficiency exceeding 98%. Irrespective of D3 treatment, the amount of Ca2+ and P(i) absorbed was positively correlated with the amount ingested, suggesting that intestinal uptake is by a passive D3-independent process. After D3 supplementation urinary Ca2+ secretion was unchanged; however, the amount of P(i) excreted in the urine increased (P less than or equal to 0.05). This resulted in a concomitant decline in P(i) AFR (P less than or equal to 0.02 from 99.95 +/- 0.02% to 99.82 +/- 0.03%). Almost all the Ca2+ and P(i) in the glomerular filtrate were reabsorbed, facilitating AFR efficiencies that approach physiological maxima (greater than 99%). Changes in AFR efficiency with D3 supplementation are therefore of no biological significance. Net mineral flux of both elements, irrespective of D3 treatment, was positive. It is speculated that the ever-growing incisors of these animals act as mineral dumps and assist in the tight regulation of plasma Ca2+ and P(i). These data suggest that naked mole rats utilize mechanisms independent of D3 in regulating mineral homeostasis and are therefore well-adapted to an environment devoid of sunlight.